Method for preparing substrate for flexible print wiring board, and substrate for flexible print wiring board

ABSTRACT

A method for preparing a substrate for a flexible print wiring board having a polyimide based resin layer wherein a solution of a polyimide based resin precursor is directly applied on an electrically conducting material to form a polyimide based resin precursor layer and then the precursor layer is cured by heating to prepare a polyimide based resin layer, characterized in that a solution of a polyimide based resin precursor B, which is one of solutions of two types of polyimide based resin precursors, is directly applied on an electrically conducting material and then, on the resultant layer is applied a solution of a polyimide based resin precursor A which allows resolving the residual strain generated in the polyimide based resin formed by the curing of the above polyimide based resin precursor B.

TECHNICAL FIELD

[0001] The present invention relates to a method for preparing asubstrate for a flexible print wiring board, and to a substrate for aflexible print wiring board. More specifically, the invention relates toa method for preparing a substrate for a flexible print wiring board,and to a substrate for a flexible print wiring board, the substratebeing free from a curl, a twist and a warp after formation of circuitrythereon and excellent in heat resistance, dimensional stability,adhesion and electrical properties.

BACKGROUND ART

[0002] Conventionally, substrates for flexible print wiring boards areproduced by combining an electrically conducting material with aninsulator such as a polyimide film or a polyester film with theintervention of an adhesive such as of an epoxy resin or an acrylicresin. However, a substrate for a substrate for a flexible print wiringboard produced by such a method is inferior in heat resistance and flameresistance because of the intervention of an adhesive layer. Inaddition, the substrate suffers from a greater dimensional variationwhen the electrically conducting material is etched or when thesubstrate is subjected to a certain heat treatment, thereby causingtrouble in subsequent steps.

[0003] To solve such drawbacks, an attempt is made to produce a flexibleprint substrate by forming a polyimide-based resin layer directly on anelectrically conducting material without the intervention of an adhesivelayer. For example, Japanese Unexamined Patent Publication No. 60-157286(1985) proposes a method for producing a substrate for a flexible printwiring board, wherein a solution of a polyimide precursor having aspecific structure is applied directly on an electrically conductingmaterial and then cured. However, when the electrically conductingmaterial is partly etched away for formation of circuitry on thesubstrate for a flexible print wiring board produced by this method, thesubstrate is significantly curled with its conductor-etched surfacefacing inward. This presents a problem that electronic components cannotaccurately be mounted on the substrate in a subsequent step, forexample, in an electronic component mounting step. To solve thisproblem, Japanese Unexamined Patent Publications No. 1-245586 (1989),No. 4-274385 (1992) and No. 8-250860 (1996) propose methods, in which alaminate consisting of a plurality of polyimide-based resin layershaving different thermal expansion coefficients is formed as theinsulative polyimide-based resin layer on the electrically conductingmaterial. However, these methods still fail to satisfactorily solve theproblem associated with the curling.

DISCLOSURE OF INVENTION

[0004] To solve the aforesaid problem, it is an object of the presentinvention to provide a method for preparing a substrate for a flexibleprint wiring board and to provide a substrate for a flexible printwiring board, the substrate being virtually free from a curl, a twistand a warp after formation of circuitry thereon and excellent in heatresistance, dimensional stability, adhesion and electrical properties.

[0005] As a result of intensive studies to solve the aforesaid problem,the inventors of the present invention have found that the intendedsubstrate for a flexible print wiring board can be obtained by employingtwo specific polyimide resin precursors for formation of a polyimideresin layer, and achieved the present invention.

[0006] To solve the aforesaid problem, a production method for asubstrate for a flexible print wiring board according to the presentinvention comprises: applying a polyimide-based resin precursor solutiondirectly on an electrically conducting material for formation of apolyimide-based resin precursor layer; and thermalcuring thepolyimide-based resin precursor layer for production of a flexible printwiring substrate having a polyimide-based resin layer; wherein asolution of one polyimide-based resin precursor (B) out of two types ofpolyimide-based resin precursors is applied directly on the electricallyconducting material; wherein a solution of the other polyimide-basedresin precursor (A) is applied on a layer of the polyimide-based resinprecursor (B), the polyimide-based resin precursor (A) being capable ofrelieving a residual strain occurring in a polyimide-based resinresulting from thermalcuring of the polyimide-based resin precursor (B).

[0007] A flexible print wring board substrate according to the presentinvention is produced by the aforesaid production method.

[0008] Thus, the residual strain occurring in the polyimide-based resinlayer due to shrinkage of the layer during the thermalcuring isrelieved, whereby a curl, a twist and a warp of the substrate can besuppressed.

[0009] A production method as set forth in claim 1 of the presentinvention comprises: applying a polyimide-based resin precursor solutiondirectly on an electrically conducting material for formation of apolyimide-based resin precursor layer; and thermalcuring thepolyimide-based resin precursor layer for production of a flexible printwiring substrate having a polyimide-based resin layer; wherein asolution of one polyimide-based resin precursor (B) out of two types ofpolyimide-based resin precursors is applied directly on the electricallyconducting material; wherein a solution of the other polyimide-basedresin precursor (A) is applied on a layer of the polyimide-based resinprecursor (B), the polyimide-based resin precursor (A) being capable ofrelieving a residual strain occurring in a polyimide-based resinresulting from thermalcuring of the polyimide-based resin precursor (B).

[0010] In a production method as set forth in claim 2 of the invention,a polyimide-based resin precursor having a higher thermalcuring ratethan the polyimide-based resin precursor (B) is employed as thepolyimide-based resin precursor (A).

[0011] In a production method as set forth in claim 3 of the invention,a polyimide-based resin precursor having a thermalcuring rate indexhigher than that of the polyimide-based resin precursor (B) by notsmaller than 10% is employed as the polyimide-based resin precursor (A).

[0012] In a production method as set forth in claim 4 of the invention,a polyimide-based resin precursor having a thermalcuring rate indexhigher than that of the polyimide-based resin precursor (B) by notsmaller than 30% is employed as the polyimide-based resin precursor (A).

[0013] In a production method as set forth in claim 5 of the invention,the polyimide-based resin precursor (B) comprises a polyamic acid, or asalt or a mixture of a diamine represented by the following structuralformula (1) and a tetracarboxylic acid derivative represented by thefollowing structural formula (2), and the polyimide-based resinprecursor (A) comprises a polyamic acid, or a salt or a mixture of adiamine represented by the following structural formula (1) and atetracarboxylic acid derivative represented by the following structuralformula (2), and the content of the tetracarboxylic acid derivative inthe polyimide-based resin precursor (A) is greater than the content ofthe tetracarboxylic acid derivative in the polyimide-based resinprecursor (B),

[0014] wherein R₁ is a tetravalent aromatic residue, R₂ is a divalentaromatic residue, and n is a real number not smaller than one whichrepresents an average number,

[0015] wherein R₃ is a tetravalent aromatic residue, and R₄ is ahydrogen atom or an alkyl group.

[0016] In a production method as set forth in claim 6 of the invention,the polyimide-based resin precursor (B) comprises a polyamic acid, atrialkylamine derivative of the polyamic acid, or a mixture of thepolyamic acid and the trialkylamine derivative, and the polyimide-basedresin precursor (A) comprises a polyamic acid, a trialkylaminederivative of the polyamic acid, or a mixture of the polyamic acid andthe trialkylamine derivative and has a greater trialkylamine contentthan the polyimide-based resin precursor (B).

[0017] A production method as set forth in claim 7 of the inventioncomprises: applying a polyimide-based resin precursor solution directlyon an electrically conducting material to form a polyimide-based resinprecursor layer; and thermalcuring the polyimide-based resin precursorlayer for production of a flexible print wiring substrate having apolyimide-based resin layer; wherein a solution of a polyimide-basedresin precursor (B) comprising a polyamic acid, or a salt or a mixtureof a diamine represented by the following structural formula (1) andpyromellitic acid or biphenyl-3,4,3′,4′-tetracarboxylic acid is applieddirectly on the electrically conducting material; wherein a solution ofa polyimide-based resin precursor (A) comprising a polyamic acid, or asalt or a mixture of a diamine represented by the following structuralformula (1) and pyromellitic acid or biphenyl-3,4,3′,4′-tetracarboxylicacid and containing pyromellitic acid orbiphenyl-3,4,3′,4′-tetracarboxylic acid in a greater amount than thepolyimide-based resin precursor (B) is applied on a layer of thepolyimide-based resin precursor (B).

[0018] wherein R₁ is a tetravalent aromatic residue, R₂ is a divalentaromatic residue, and n is a real number not smaller than one whichrepresents an average number.

[0019] A production method as set forth in claim 8 of the inventioncomprises: applying a polyimide-based resin precursor solution directlyon an electrically conducting material to form a polyimide-based resinprecursor layer; and thermalcuring the polyimide-based resin precursorlayer for production of a flexible print wiring substrate having apolyimide-based resin layer; wherein a solution of a polyimide-basedresin precursor (B) comprising a polyamic acid, a triethylaminederivative of the polyamic acid or a mixture of the polyamic acid andthe triethylamine derivative is applied directly on the electricallyconducting material; wherein a solution of a polyimide-based resinprecursor (A) comprising a polyamic acid, a triethylamine derivative ofthe polyamic acid or a mixture of the polyamic acid and thetriethylamine derivative and containing triethylamine in a greateramount than the polyimide-based resin precursor (B) is applied on alayer of the polyimide-based resin precursor (B).

[0020] A substrate for a flexible print wiring board as set forth inclaim 9 of the invention is produced by a production method as recitedin any of claims 1 to 6.

[0021] A substrate for a flexible print wiring board as set forth inclaim 10 of the invention is produced by a production method as recitedin claim 7 or 8.

[0022] The present invention will hereinafter be described in detail.

[0023] In the following explanation, the polyimide-based resin precursor(A), the polyimide-based resin precursor (B), the solution of thepolyimide-based resin precursor (A) and the solution of thepolyimide-based resin precursor (B) are referred to simply as “precursor(A)”, “precursor (B)”, “solution (A)” and “solution (B)”, respectively.

[0024] In the present invention, the polyimide-based resins are intendedto include heat resistant resins such as polyimides, polyamide-imides,polybenzimidazoles and polyimide esters. The polyimide-based resinspreferably each comprise a polyimide resin as a main component thereof.Therefore, the polyimide-based resin precursors are imidized into thepolyimide-based resins.

[0025] In the present invention, a polyimide-based resin precursorhaving a higher thermalcuring rate than the precursor (B) is preferablyemployed as the precursor (A).

[0026] The thermalcuring rate of the polyimide-based resin precursor isdetermined, for example, in the following manner.

[0027] A varnish of the polyimide-based resin precursor is applied on a3-mm thick glass plate, and heat-treated at 350° C. for two hours. Then,the resulting coated film is peeled off at a room temperature, andsufficiently pulverized in an agate mortar. The resulting powder ishomogeneously mixed with fine KBr powder in the mortar, and theresulting mixture is pressed into a KBr disk by means of a pressmachine. After the KBr disk is dried in a desiccator for not shorterthan 12 hours, an infrared spectrum is obtained with the use of the KBrdisk by means of a Fourier transformation infrared spectrophotometerSystem-2000 (with a TGS detector for 64-time integration) available fromthe Perkin-Elmer Corporation.

[0028] That is, an absorbance a₁ based on absorption at 1770 cm⁻¹attributable to imide carbonyl bonds and an absorbance a₂ based onabsorption at 1500 cm⁻¹ attributable to C—H bonds of a benzene ring aredetermined, and an absorbance ratio a is calculated from the followingexpression:

a=absorbance a ₁/absorbance a₂

[0029] Then, an infrared spectrum is obtained in substantially the samemanner as described above, except that the heat treatment is performedat a temperature of 160° C. for 10 minutes. An absorbance b₁ based onabsorption at 1770 cm⁻¹ attributable to imide carbonyl bonds and anabsorbance b₂ based on absorption at 1500 cm⁻¹ attributable to C—H bondsof a benzene ring are determined, and an absorbance ratio b iscalculated from the following expression:

b=absorbance b ₁/absorbance b₂

[0030] Thereafter, a thermalcuring rate index c is calculated from(a/b)×100(%). The thermalcuring rate index c indicates the imide ringopening ratio of the polyimide-based resin precursor. The higher thevalue of the thermalcuring rate index, the higher the thermalcuring rateof the polyimide-based resin precursor.

[0031] In the present invention, a polyimide-based resin precursorhaving a thermalcuring rate index higher than that of the precursor (B)by not smaller than 10% is preferably employed as the precursor (A).

[0032] In the present invention, a polyimide-based resin precursorhaving a thermalcuring rate index higher than that of the precursor (B)by not smaller than 30% is more preferably employed as the precursor(A).

[0033] In the present invention, preferred examples of thepolyimide-based resin precursors are as follows.

[0034] A polyimide-based resin precursor comprising a polyamic acid, atrialkylamine derivative of the polyamic acid or a mixture of thepolyamic acid and the trialkylamine derivative is employed as theprecursor (B). A polyimide-based resin precursor comprising a polyamicacid, a trialkylamine derivative of the polyamic acid or a mixture ofthe polyamic acid and the trialkylamine derivative and having a greatertrialkylamine content than the precursor (B) is employed as theprecursor (A). The precursor (B) may contain no trialkylamine.

[0035] Alkyl groups in the trialkylamine derivative of the polyamic acidpreferably each have a carbon number of 1 to 10. The alkyl groups in thetrialkylamine derivative may be linear, branched or cyclic. The threealkyl groups may be the same or different.

[0036] Examples of the trialkylamine derivative of the polyamic acidinclude a trimethylamine derivative, a triethylamine derivative, atriisopropylamine derivative, a tri(n-propyl)amine derivative, atriisobutylamine derivative, a tri(n-butyl)amine derivative, atri(t-butyl)amine derivative, a cyclohexyldimethylamine derivative and abutyldimethylamine derivative of the polyamic acid, but not limitedthereto. The trialkylamine preferably has a boiling point of not higherthan 200° C. so as to evaporate together with a solvent at imidization.

[0037] Other preferred examples of the polyimide-based resin precursorsare as follows.

[0038] A polyimide-based resin precursor comprising a polyamic acid, ora salt or a mixture of a diamine represented by the aforesaid structuralformula (1) and a tetracarboxylic acid derivative represented by theaforesaid structural formula (2) is employed as the precursor (B). Apolyimide-based resin precursor comprising a polyamic acid, or a salt ora mixture of a diamine represented by the aforesaid structural formula(1) and a tetracarboxylic acid derivative represented by the aforesaidstructural formula (2) and containing the tetracarboxylic acidderivative in a greater amount than the precursor (B) is employed as theprecursor (A). The precursor (B) may contain no tetracarboxylic acidderivative.

[0039] Usable as the electrically conducting material in the presentinvention are metal foils made from metals such as copper, aluminum,iron, silver, palladium, nickel, chromium, molybdenum, tungsten, andalloys of any of these metals. Among these metal foils, a copper foil ismost preferable.

[0040] The electrically conducting material may be subjected to achemical or mechanical surface treatment for improvement of the adhesionthereof to the polyimide-based resin. Examples of the chemical surfacetreatment include plating treatments such as plating with nickel andplating with a copper-zinc alloy, and treatments with an aluminumalcoholate, an aluminum chelate and a silane coupling agent. A silanecoupling agent having an amino group is preferably employed as thesilane coupling agent. An exemplary mechanical surface treatment issand-blasting.

[0041] The polyimide-based resin precursors are applied as solutionsthereof on the electrically conducting material. Exemplary solvents tobe employed for dissolving the polyimide-based resin precursors includenon-protonic polar solvents, ether compounds and water-soluble alcoholcompounds.

[0042] Examples of the non-protonic polar solvents includeN-methyl-2-pyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide,dimethyl sulfoxide and hexamethylphosphoramide.

[0043] Examples of the ether compounds include 2-methoxyethanol,2-ethoxyethanol, 2-(methoxymethoxy)ethoxyethanol, 2-isopropoxyethanol,2-butoxyethanol, tetrahydrofurfuryl alcohol, diethylene glycol,diethylene glycol monomethyl ether, diethylene glycol monoethyl ether,diethylene glycol monobutyl ether, triethylene glycol, triethyleneglycol monoethyl ether, tetraethylene glycol, 1-methoxy-2-propanol,1-ethoxy-2-propanol, dipropylene glycol, dipropylene glycol monomethylether, dipropylene glycol monoethyl ether, tripropylene glycolmonomethyl ether, polyethylene glycol, polypropylene glycol,tetrahydrofuran, dioxane, 1,2-dimethoxyethane, diethylene glycoldimethyl ether, and diethylene glycol diethyl ether.

[0044] Examples of the water-soluble alcohol compounds include methanol,ethanol, 1-propanol, 2-propanol, tert-butyl alcohol, ethylene glycol,1,2-propandiol, 1,3-propandiol, 1,3-butandiol, 1,4-butandiol,2,3-butandiol, 1,5-pentandiol, 2-buten-1,4-diol,2-methyl-2,4-pentandiol, 1,2,6-hexantriol, and diacetone alcohol.

[0045] These solvents may be used as a mixture of two or more of them.Among these solvents, it is preferred to use N,N-dimethylacetamide orN-methyl-2-pyrrolidone alone. It is also preferred to use a combinationof N,N-dimethylacetamide and N-methyl-2-pyrrolidone, a combination ofN-methyl-2-pyrrolidone and methanol, or a combination ofN-methyl-2-pyrrolidone and 2-methoxyethanol as a solvent mixture.

[0046] Next, an explanation will be given to how to prepare thepolyimide-based resin precursors.

[0047] First, the solution of the polyamic acid is prepared by allowingan aromatic tetracarboxylic dianhydride represented by the followingstructural formula (3) and an aromatic diamine represented by thefollowing structural formula (4) to react with each other in any of theaforesaid solvents, e.g., in a non-protonic polar solvent.

[0048] wherein R₁ is a tetravalent aromatic residue,

H₂N—R₂—NH₂  (4)

[0049] wherein R₂ is a divalent aromatic residue.

[0050] For the aforesaid reaction, the proportion of the tetracarboxylicdianhydride is preferably 1.03 to 0.97 mol, more preferably 1.01 to 0.99mol, based on 1 mol of the diamine. A reaction temperature is preferably−30 to 60° C., more preferably −20 to 40° C.

[0051] In the aforesaid reaction, the mixing order of the monomers andthe solvent is not particularly limited, but the monomers and thesolvent may be mixed in any order. Where a solvent mixture is employedas the solvent, the solution of the polyamic acid is obtained bydissolving or dispersing the respective monomers in different solvents,mixing the resulting solutions or dispersions together, and allowing themonomers to react with each other with stirring at a predeterminedtemperature for a predetermined period.

[0052] The trialkylamine derivative of the polyamic acid is obtained,for example, by adding 0.2 to 1.0 mol, preferably 0.3 to 0.8 mol, of thetrialkylamine based on 1 mol of carboxyl groups of the polyamic acid tothe solution of the polyamic acid in the non-protonic polar solventobtained in the aforesaid manner, and allowing the trialkylamine and thepolyamic acid to react with each other at 10 to 90° C., preferably at 20to 80° C. Alternatively, the trialkylamine derivative of the polyamicacid can be obtained by addition of the trialkylamine during thepreparation of the polyamic acid.

[0053] The solution (A) and the solution (B) are different in thecontent of the trialkylamine derivative. The precursor (A) is preferablypresent in a greater proportion than the precursor (B) by not smallerthan 1% by mass, more preferably not smaller than 2% by mass, based onthe total weight of the precursors (A) and (B).

[0054] The solution of the polyimide-based resin precursor comprisingthe salt of the diamine represented by the structural formula (1) andthe tetracarboxylic acid derivative represented by the structuralformula (2) is prepared by adding the tetracarboxylic acid derivativerepresented by the structural formula (2) to the solution of the diaminerepresented by the structural formula (1).

[0055] An explanation will be given to a preferred example of a methodfor preparing the polyimide resin precursor solution by allowing thetetracarboxylic dianhydride and the diamine to react with each other inthe non-protonic polar solvent to prepare the solution of the diaminerepresented by the structural formula (1), and adding thetetracarboxylic acid derivative represented by the structural formula(2) to the diamine solution.

[0056] First, the aromatic tetracarboxylic dianhydride represented bythe structural formula (3) and the aromatic diamine represented by thestructural formula (4) are allowed to react with each other in thenon-protonic polar solvent to prepare the solution of the diaminerepresented by the structural formula (1).

[0057] For the reaction of the tetracarboxylic dianhydride and thediamine for the preparation of the diamine represented by the structuralformula (1), the proportion of the tetracarboxylic dianhydride ispreferably 0.50 to 0.95 mol, more preferably 0.60 to 0.90 mol, based on1 mol of the diamine. A reaction temperature is preferably −30 to 60°C., more preferably −20 to 40° C.

[0058] Then, the aromatic tetracarboxylic acid derivative represented bythe structural formula (2) is added to the solution of the diaminerepresented by the structural formula (1), whereby the solution of thepolyimide resin precursor comprising the salt of the diamine and thearomatic tetracarboxylic acid derivative is prepared.

[0059] The aromatic tetracarboxylic acid derivative represented by thestructural formula (2) is preferably added in a proportion of 0.97/2 to1.03/2 mol, more preferably 0.99/2 to 1.01/2 mol, based on oneequivalent of amino groups in the diamine represented by the structuralformula (1).

[0060] When the solution of the diamine represented by the structuralformula (1) is prepared, a diamine may be used alone or two types ofdiamines may be used as a mixture. The mixing order of the monomers ofthe aforesaid aromatic tetracarboxylic dianhydride and the aforesaidaromatic diamine and the solvent is not particularly limited, but themonomers and the solvent may be mixed in any order. Where a solventmixture is employed as the solvent, the solution of the diaminerepresented by the structural formula (1) is prepared by dissolving ordispersing the respective monomers in different solvents, mixing theresulting solutions or dispersions together, and allowing the monomersto react with each other with stirring at a predetermined temperaturefor a predetermined period.

[0061] The tetracarboxylic acid derivative represented by the structuralformula (2) may be added in a solid or solution form to the aforesaiddiamine solution with stirring. The tetracarboxylic acid derivatives maybe used either alone or as a mixture of two of them.

[0062] Two or more types of the polyimide resin precursor solutions maybe used as a mixture.

[0063] Specific examples of the aromatic tetracarboxylic acid derivativerepresented by the structural formula (2) include:

[0064] pyromellitic acid;

[0065] biphenyl-3,3′,4,4′-tetracarboxylic acid;

[0066] benzophenone-3,3′,4,4′-tetracarboxylic acid;

[0067] diphenyl sulfone-3,3′,4,4′-tetracarboxylic acid;

[0068] diphenyl ether-2,3,3′,4′-tetracarboxylic acid;

[0069] benzophenone-2,3,3′,4′-tetracarboxylic acid;

[0070] naphthalene-2,3,6,7-tetracarboxylic acid;

[0071] naphthalene-1,4,5,7-tetracarboxylic acid;

[0072] naphthalene-1,2,5,6-tetracarboxylic acid;

[0073] diphenylmethane-3,3′,4,4′-tetracarboxylic acid;

[0074] 2,2-bis(3,4-dicarboxyphenyl)propane;

[0075] 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane;

[0076] 3,4,9,10-tetracarboxyperylene;

[0077] 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane;

[0078] 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl] hexafluoropropane; and

[0079] dimethyl esters, diethyl esters and dipropyl esters of theseacids. The aromatic tetracarboxylic acid derivatives may be used as amixture of two or more of them.

[0080] Specific examples of the aromatic tetracarboxylic dianhydriderepresented by the structural formula (3) include dianhydrides of theaforesaid aromatic tetracarboxylic acids. These aromatic tetracarboxylicdianhydrides may be used as a mixture of two or more of them.

[0081] Specific examples of the aromatic diamine represented by thestructural formula (4) include:

[0082] p-phenylenediamine;

[0083] m-phenylenediamine;

[0084] 3,4′-diaminodiphenyl ether;

[0085] 4,4′-diaminodiphenyl ether;

[0086] 4,4′-diaminodiphenylmethane;

[0087] 3,3′-dimethyl-4,4′-diaminodiphenylmethane;

[0088] 2,2-bis[4-(4-aminophenoxy)phenyl]propane;.

[0089] 1,2-bis(anilino)ethane;

[0090] diaminodiphenyl sulfone;

[0091] diaminobenzanilide;

[0092] diaminobenzoate;

[0093] diaminodiphenyl sulfide;

[0094] 2,2-bis(p-aminophenyl)propane;

[0095] 2,2-bis(p-aminophenyl)hexafluoropropane;

[0096] 1,5-diaminonaphthalene;

[0097] diaminotoluene;

[0098] diaminobenzotrifluoride;

[0099] 1,4-bis(p-aminophenoxy)benzene;

[0100] 4,4′-bis(p-aminophenoxy)biphenyl;

[0101] diaminoanthraquinone;

[0102] 4,4′-bis(3-aminophenoxyphenyl)diphenyl sulfone;

[0103] 1,3-bis(anilino)hexafluoropropane;

[0104] 1,4-bis(anilino)octafluorobutane;

[0105] 1,5-bis(anilino)decafluoropentane; and

[0106] 1,7-bis(anilino)tetradecafluoroheptane.

[0107] These aromatic diamines may be used as a mixture of two or moreof them.

[0108] The solution (A) and the solution (B) are different in thecontent of the aromatic tetracarboxylic acid derivative. The precursor(A) is preferably present in a greater proportion than the precursor (B)by not smaller than 1% by mass, more preferably not smaller than 2% bymass, based on the total weight of the precursors (A) and (B).

[0109] In any of the aforesaid embodiments of the present invention, aderivative of an amine, a diamine, a dicarboxylic acid, a tricarboxylicacid and a tetracarboxylic acid having unsaturated bonds mayadditionally be employed for forming a crosslinked structure during thethermalcuring when the polyimide resin precursor solution is prepared.Usable as the unsaturated compounds are maleic acid, nadic acid,tetrahydrophthalic acid and ethynylaniline.

[0110] Even where the polyimide resin precursors are partially imidizeddepending on the conditions for preparing and drying the polyimide resinprecursor and the like, no particular problem arises.

[0111] When the polyimide resin precursor solutions are prepared, apolyimide resin, a polyamide-imide resin or other heat-resistant resinsoluble in the aforesaid solvent may be mixed with the solutions.Further, a silane coupling agent and any of various surface-activeagents may be added in a very small amount to the solutions forimprovement of adhesion (adhesiveness) and film properties.

[0112] Next, an explanation will be given to a production method for aflexible print wiring substrate according to the present invention.

[0113] The solution (A) and the solution (B) prepared according to eachof the aforesaid embodiments are applied on the electrically conductingmaterial, and dried for formation of precursor layers, which are thenthermally cured to be imidized. Thus, the polyimide resin layer isformed which includes a coated film (A) resulting from the imidizationof the precursor (A) contained in the solution (A) and a coated film (B)resulting from the imidization of the precursor (B) contained in thesolution (B).

[0114] For the formation of the polyimide resin layer, the solution (B)is applied directly on the electrically conducting material, and thesolution (A) is applied on a layer of the solution (B).

[0115] More specifically, the solution (B) is applied on a roughenedsurface of the electrically conducting material having a predeterminedthickness so as to allow the coated film (B) to have a predeterminedthickness after the thermalcuring. The applied solution (B) ispreferably dried at a temperature of not higher than 200° C., morepreferably not higher than 150° C. for formation of a precursor coatedfilm as an inner layer. Further, the solution (A) is applied on theprecursor coated film so as to allow the coated film (A) to have apredetermined thickness after the thermalcuring. The applied solution(A) is preferably dried at a temperature of not higher than 200° C.,more preferably not higher than 150° C. for formation of a precursorcoated film as an outer layer. Finally, a heat treatment is performed ata temperature not lower than 150° C. and not higher than 500° C.,whereby the two precursor coated films are thermally cured to beimidized. Thus, the substrate for a flexible print wiring board isobtained which includes the coated film (A) and the coated film (B)formed as the insulative polyimide-based resin layer on the electricallyconducting material.

[0116] The application of the solution (A) and the solution (B) may becarried out a plurality of times, and then the resulting coated filmsmay be thermally cured. Further, two or more polyimide-based resinlayers each including the coated film (B) and the coated film (A) may beformed.

[0117] Although polyimide resin precursors have been explained as thepolyimide-based resin precursors, the substrate for a flexible printwiring board can be produced in the same manner by employing otherpolyimide-based resin precursors.

[0118] Further, a layer of another polyimide-based resin or any otherheat-resistant resin may additionally be formed between the electricallyconducting material and the polyimide-based resin layer for improvementof the adhesion (adhesiveness) or coating properties.

[0119] Where the two types of solutions (A) and (B) are applied on theelectrically conducting material in the present invention, the thickness(t1) of the coated film (A) and the thickness (t2) of the coated film(B) after the thermalcuring may be the same, but the thickness ratio(t2/t1) is preferably 0.01 to 100, more preferably 0.1 to 10, furthermore preferably 0.3 to 3. The thickness (t1+t2) of the entirepolyimide-based resin layer is typically 5 to 100 μm, preferably 10 to50 μm.

[0120] Further, the polyimide-based resin layer preferably has anaverage linear expansion coefficient of 10 to 40 ppm. As long as theaverage linear expansion coefficient is within this range, the chemicalstructures of the polyimide-based resins of the two coated films or thelinear expansion coefficients of the two polyimide-based resins of thecoated films which depend on the chemical structures may be the same ordifferent. Where the polyimide-based resin layers have different linearexpansion coefficients, the linear expansion coefficient of the coatedfilm (B) in contact with the electrically conducting material may behigher or lower than the linear expansion coefficient of the coated film(A).

[0121] In the present invention, a polyimide-based resin precursor,which allows resolving a residual strain generated in thepolyimide-based resin layer formed by the thermalcuring of the precursor(B), is employed as the precursor (A). In other words, a residual strainof the polyimide-based resin formed by the thermalcuring of theprecursor (B), which is generated between the precursor (B) and theelectrically conducting material, is counterbalanced with that of thepolyimide-based resin formed by the thermalcuring of the precursor (A),which is generated between the precursor (A) and the precursor (B).Thus, the residual strain generated in the polyimide-based resin betweenthe electrically conducting material and the polyimide-based resin layeris resoled, so that a curl, a twist and a warp can be suppressed.Further, the substrate is improved in dimensional stability andelectrical properties such as an insulation breakdown voltage, andexcellent in heat resistance, adhesion, flexural resistance and chemicalresistance.

[0122] In the present invention, the optimum thicknesses of the layersof the precursors (A) and (B) formed on the electrically conductingmaterial are determined by a simple trial-and-error method, though theoptimum thicknesses depend on the basic chemical structures of thepolyimide-based resin precursors. Where the curl of the substrate is notsufficiently suppressed after the etching of the substrate, for example,the thicknesses of the precursor layers to be stacked are simplyincreased or reduced for adjustment.

[0123] For the application of the polyimide-based resin precursors onthe electrically conducting material, a die coater, a multi-layer diecoater, a gravure coater, a comma coater, a reverse roll coater, adoctor blade coater or the like can be employed as an industrial coatingmachine. For the thermalcuring of the applied precursors, a copper foilon which the precursors are applied is wound up into a roll and thenheated in an inert gas atmosphere in an oven, or a heating zone isprovided in a production line.

[0124] In the production method according to the present invention, thepolyimide-based resin layer is formed from the specific polyimide-basedresin precursors as described above, so that the curl, the twist and thewarp are suppressed after formation of circuitry. In addition, thesubstrate for a flexible print wiring board is excellent in heatresistance, dimensional stability at high temperatures, adhesion andelectrical properties.

[0125] Since the curl, twist and warp of the substrate for a flexibleprint wiring board are suppressed according the present invention,electronic components can be mounted at a higher integration density onthe substrate for a flexible print wiring board without any trouble.Further, the substrate for a flexible print wiring board is excellent inheat resistance, dimensional stability at high temperatures, adhesionand electrical properties.

EXAMPLES

[0126] The present invention will hereinafter be described morespecifically by way of examples thereof. However, it should beunderstood that the invention not be limited to the examples.

[0127] Reference Examples 1 to 10 show preparation of polyimide resinprecursor solutions. Compounds employed in Reference Examples 1 to 10are abbreviated as follows.

[0128] (Reaction Components) BPDA: biphenyl-3,3′,4,4′-tetracarboxylicdianhydride BPA-A: biphenyl-3,3′,4,4′-tetracarboxylic acid BPA-E:dimethyl biphenyl-3,3′,4,4′-tetracarboxylate PMDA: pyromelliticdianhydride ODA: 4,4′-oxydianiline PDA: p-phenylenediamine

[0129] (Solvents) DMAc: N,N-dimethylacetamide NMP:N-methyl-2-pyrrolidone

Reference Example 1

[0130] First, 30.03 g (0.15 mol) of ODA, 91.92 g (0.85 mol) of PDA, 2330g of DMAc and 999 g of NMP were put into a three-neck flask in a streamof nitrogen gas, and stirred for 30 minutes in the flask in ice water.Then, 294.22 g (1.00 mol) of BPDA was added to the flask, and theresulting mixture was stirred for one hour in the flask in a 40° C.water bath. Thus, a homogeneous polyimide resin precursor solutioncontaining a polyamic acid was prepared. This solution was defined as aprecursor solution (a).

Reference Example 2

[0131] First, 13.34 g (123.3 mmol) of PDA and 4.36 g (21.8 mmol) of ODAwere put into a three-neck flask in a stream of nitrogen gas, then 245 gof DMAc and 105 g of NMP were added to the flask, and the resultingmixture was stirred. Then, 32.3 g (148.1 mmol) of PMDA was added to theflask, and the resulting mixture was stirred at a room temperature (25°C.) overnight (for 12 hours). Thus, a homogeneous polyamic acid solutionhaving a solid concentration of 12.5% by mass was prepared. Thissolution was defined as a precursor solution (b).

Reference Example 3

[0132] First, 18.38 g (62.5 mmol) of BPDA was put into a three-neckflask in a stream of nitrogen gas, and then 122.5 g of DMAc was added tothe flask for dissolution of BPDA. Then, 6.62 g (61.2 mmol) of PDA and52.5 g of NMP were added to the flask, and the resulting mixture wasstirred at a room temperature overnight. Thus, a homogeneous polyamicacid solution having a solid concentration of 12.5% by mass wasprepared. This solution was defined as a precursor solution (c).

[0133] The precursor solutions (a) to (c) prepared in Reference Examples1 to 3 contained no tetracarboxylic acid derivative, and the reactioncomponents contained no trialkylamine. Therefore, the trialkylaminecontent was 0% by mass.

Reference Example 4

[0134] First, 81.0 g (0.80 mol) of triethylamine was slowly addeddropwise to the polyamic acid solution (precursor solution (a)) preparedin Reference Example 1, and the resulting mixture was stirred at a roomtemperature for three hours. Thus, a solution of a triethylamine salt ofthe polyamic acid was prepared. This solution was defined as a precursorsolution (d). The triethylamine content of the precursor solution was2.1% by mass.

Reference Example 5

[0135] First, 202.38 g (2.0 mol) of triethylamine was slowly addeddropwise to the polyamic acid solution (precursor solution (a)) preparedin Reference Example 1, and the resulting mixture was stirred at a roomtemperature for three hours. Thus, a solution of a triethylamine salt ofthe polyamic acid was prepared. This solution was defined as a precursorsolution (e). The triethylamine content of the precursor solution was5.1% by mass.

Reference Example 6

[0136] First, 12.65 g (0.125 mol) of triethylamine was slowly addeddropwise to the polyamic acid solution (precursor solution (c)) preparedin Reference Example 3, and the resulting mixture was stirred at roomtemperature for three hours. Thus, a solution of a triethylamine salt ofthe polyamic acid was prepared. This solution was defined as a precursorsolution (f). The triethylamine content of the precursor solution was5.9% by mass.

Reference Example 7

[0137] First, 30.03 g (0.15 mol) of ODA, 91.92 g (0.85 mol) of PDA, 1180g of DMAc and 506 g of NMP were put into a three-neck flask in a streamof nitrogen gas, and stirred for 30 minutes in the flask in ice water.Then, 250.09 g (0.85 mol) of BPDA was added to the flask, and theresulting mixture was stirred for one hour in a 40° C. water bath.Subsequently, 49.54 (0.15 mol) of BPA-A was added to the flask, and theresulting mixture was stirred in a 40° C. water bath for two hours andthen in a 60° C. water bath for three hours. Thus, a homogeneouspolyimide resin precursor solution containing a salt of a diamine and atetracarboxylic acid was prepared. This solution was defined as aprecursor solution (g). The content of a tetracarboxylic acid derivativein the precursor solution was 11.8% by mass.

Reference Example 8

[0138] A polyimide resin precursor solution was prepared insubstantially the same manner as in Reference Example 7, except that220.67 g (0.75 mol) of BPDA, 82.56 g (0.25 mol) of BPA-A, 992 g of DMAcand 425 g of NMP were employed. This solution was defined as a precursorsolution (h). The content of a tetracarboxylic acid derivative in theprecursor solution was 19.4% by mass.

Reference Example 9

[0139] A polyamide resin precursor solution was prepared insubstantially the same manner as in Reference Example 7, except thatBPA-E was employed instead of BPA-A. This solution was defined as aprecursor solution (i). The content of a dimethyl tetracarboxylate inthe precursor solution was 0.8% by mass.

Reference Example 10

[0140] The precursor solution (a) and the precursor solution (h) wereblended in amounts such as to contain equivalent amounts of thepolyimide resin precursors. Thus, a homogeneous polyimide resinprecursor solution was prepared. This solution was defined as aprecursor solution (j). The content of a tetracarboxylic acid derivativein the precursor solution was 9.7% by mass.

[0141] The thermalcuring rate indices of the polyimide resin precursorscontained in the precursor solutions (a) to (j) prepared in ReferenceExamples 1 to 10 were determined. In Table 1, the results are showntogether with the content of the tetracarboxylic acid derivative or thetrialkylamine. TABLE 1 Content of tetracarboxylic Content of ReferencePrecursor acid derivative Trialkylamine Thermalcuring Example solution(% by mass) (% by mass) rate index (%) 1 (a) 0 0 16 2 (b) 0 0 19 3 (c) 00 13 4 (d) 0 2.1 27 5 (e) 0 5.1 34 6 (f) 0 5.9 28 7 (g) 11.8 — 67 8 (h)19.4 — 80 9 (i) 20.8 — 82 10 (j) 9.7 — 55

Example 1

[0142] A 35-μm thick copper foil produced by electrolysis was fixed in ametal frame. The precursor solution (a) was applied on the copper foilby means of a bar coater to such a thickness that the resulting coatedfilm (B) had a thickness of 5 μm after thermalcuring. Then, the appliedsolution was dried at 130° C. for ten minutes for formation of an innerlayer. Subsequently, the precursor solution (g) was applied on the innerlayer in a room temperature atmosphere (25° C.) by means of a bar coaterto such a thickness that the resulting coated film (A) had a thicknessof 20 μm after thermalcuring. Then, the applied solution was dried at100° C. for five minutes for formation of an outer layer. The inner andouter layers were subjected to a heat treatment by increasing thetemperature from 100° C. to 360° C. in two hours and then keeping thetemperature at 360° C. for two hours, whereby the precursor in the innerlayer and the precursor in the outer layer were thermally cured to beimidized. Thus, a substrate for a flexible print wiring board (coppertensile board) was produced, which had a polyimide resin layer includingthe coated film (B) and the coated film (A) formed on the coated film(B).

Example 2

[0143] A substrate for a flexible print wiring board was produced in thesame manner as in Example 1, except that the precursor solution (h) wasemployed instead of the precursor solution (g) and the coated film (B)and the coated film (A) respectively had thicknesses of 17 μm and 8 μm.

[0144] Adhesion between the copper foil and the resin layer of thesubstrate for a flexible print wiring board was measured by means ofTENSILONTESTER (a versatile precision material tester MODEL-2020available from Intesco).

[0145] For the measurement of the adhesion, the substrates were each cutinto a 10-mm wide strip, which was fixed to an aluminum plate with itsresin layer in contact with the aluminum plate by a two-sided adhesivetape having an adhesive applied on opposite sides thereof. In thisstate, the copper foil was peeled apart from the resin layer in a180-degree direction at 50 mm/minute for determination of the adhesion.The result is excellent with an adhesion of 1.6 kg/cm.

[0146] Further, the substrate for a flexible print wiring board wasimmersed in an aqueous solution of ferric chloride, so that the copperfoil was entirely etched away from the substrate by the ferric chlorideaqueous solution. The linear expansion coefficient and second-ordertransition temperature Tg of the resin layer obtained after the etchingwere determined by means of a thermo-mechanical analyzer (TMA: TMA2940available from TA Instruments). As a result, the linear expansioncoefficient was 23 ppm, and the second-order transition temperature Tgwas 345° C. It was found that the resin layer was excellent indimensional stability and heat resistance.

Example 3

[0147] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (i) was employed instead of the precursor solution (g) and thecoated film (B) and the coated film (A) respectively had thicknesses of14 μm and 11 μm.

Example 4

[0148] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (j) was employed instead of the precursor solution (g) and thecoated film (B) and the coated film (A) respectively had thicknesses of4 μm and 21 μm.

Example 5

[0149] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (j) and the precursor solution (h) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 12 μm and 9 μm.

Example 6

[0150] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (b) and the precursor solution (d) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 14 μm and 11 μm.

Example 7

[0151] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (c) and the precursor solution (e) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 14 μm and 11 μm.

Example 8

[0152] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 5, except that the coatedfilm (A) had a thickness of 15 μm.

Example 9

[0153] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (e) was employed instead of the precursor solution (g) and thecoated film (B) and the coated film (A) respectively had thicknesses of11 μm and 14 μm.

Example 10

[0154] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (c) and the precursor solution (f) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 5 μm and 20 μm.

Comparative Example 1

[0155] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (a) was employed instead of the precursor solution (g), and thecoated film (B) and the coated film (A) respectively had thicknesses of10 μm and 15 μm.

Comparative Example 2

[0156] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (g) and the precursor solution (a) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 10 μm and 15 μm.

Comparative Example 3

[0157] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (h) and the precursor solution (a) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 10 μm and 15 μm.

Comparative Example 4

[0158] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (j) and the precursor solution (a) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 10 μm and 15 μm.

Comparative Example 5

[0159] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (e) and the precursor solution (a) were employed instead of theprecursor solution (a) and the precursor solution (g), respectively, andthe coated film (B) and the coated film (A) respectively had thicknessesof 10 μm and 15 μm.

Comparative Example 6

[0160] A substrate for a flexible print wiring board was produced insubstantially the same manner as in Example 1, except that the precursorsolution (c) was employed instead of the precursor solution (g) and thecoated film (B) and the coated film (A) respectively had thicknesses of14 μm and 11 μm.

[0161] The precursor solutions applied for the formation of the innerlayer and the outer layer and the thicknesses of the coated films inExamples 1 to 10 and Comparative Examples 1 to 6 are collectively shownin Table 2. TABLE 2 Precursor solution Thickness of Coated film (μm)Inner layer Outer layer B A Example 1 a g 5 20 2 a h 17 8 3 a i 14 11 4a j 4 21 5 j h 12 9 6 b d 14 11 7 c e 14 11 8 j h 12 15 9 a e 11 14 10 cf 5 20 Comparative Example 1 a a 10 15 2 g a 10 15 3 h a 10 15 4 j a 1015 5 e a 10 15 6 a c 14 11

[0162] Further, the curl resistant properties of the substrate for aflexible print wiring boards obtained in Examples 1 to 10 andComparative Examples 1 to 6 (pre-etching) and the curl resistantproperties of the resin layers obtained by entirely etching away thecopper foils from the substrates in the ferric chloride aqueous solution(post-etching) were determined. The results are shown in Table 3.

[0163] For the determination of the curl resistant properties, teststrips having a length of 10 cm and a width of 10 cm were dried at 100°C. for 10 minutes, and the curvature radii of the resulting curls of thetest strips were measured. A symbol X indicates that the curvatureradius of the curl was smaller than 20 mm, a symbol Δ indicates that thecurvature radius was smaller than 50 mm and not smaller than 20 mm, asymbol ◯ indicates that the curvature radius was smaller than 90 mm andnot smaller than 50 mm, and a symbol ⊚ indicates that the curvatureradius was not smaller than 90 mm. TABLE 3 Curl resistant propertiesExample Pre-etching Post-etching 1 ◯ ⊚ 2 ◯ ⊚ 3 ◯ ⊚ 4 ◯ ⊚ 5 ◯ ⊚ 6 ◯ ◯ 7 ◯◯ 8 ◯ ⊚ 9 ◯ ◯ 10  ◯ ◯ Comparative Curl resistant properties ExamplePre-etching Post-etching 1 ◯ x 2 ◯ x 3 ◯ x 4 ◯ x 5 ◯ Δ 6 ◯ Δ

[0164] As shown in Table 3, the substrate for a flexible print wiringboards of Examples 1 to 10 each having an outer layer formed of aprecursor having a higher thermalcuring rate index than that of an innerlayer are less liable to be curled with excellent curl resistantproperties. Therefore, electronic components can be mounted at a higherintegration density on the substrate for a flexible print wiring boards.On the other hand, the substrate for a flexible print wiring board ofComparative Example 1 having an inner layer and an outer layer formed ofprecursors having the same thermalcuring rate indices, and the substratefor a flexible print wiring boards of Comparative Examples 2 to 6 eachhaving an inner layer formed of a precursor having a higherthermalcuring rate index than that of an outer layer are more liable tobe curled with poor curl resistant properties.

[0165] As apparent from Example 2, the substrate for a flexible printwiring board produced in accordance with the inventive production methodis excellent in heat resistance, dimensional stability, and adhesionbetween the copper foil and the resin layer.

1. A production method for a substrate for a flexible print wiringboard, the method comprising: applying a polyimide-based resin precursorsolution directly on an electrically conducting material for formationof a polyimide-based resin precursor layer; and thermally curing thepolyimide-based resin precursor layer for production of a flexible printwiring substrate having a polyimide-based resin layer; wherein asolution of one polyimide-based resin precursor (B) out of two types ofpolyimide-based resin precursors is applied directly on the electricallyconducting material; wherein a solution of the other polyimide-basedresin precursor (A) is applied on a layer of the polyimide-based resinprecursor (B), the polyimide-based resin precursor (A) being capable ofresolving a residual strain generated in a polyimide-based resin formedby the thermalcuring of the polyimide-based resin precursor (B).
 2. Aproduction method for a substrate for a flexible print wiring board asset forth in claim 1, wherein a polyimide-based resin precursor having ahigher thermalcuring rate than the polyimide-based resin precursor (B)is employed as the polyimide-based resin precursor (A).
 3. A productionmethod for a substrate for a flexible print wiring board as set forth inclaim 2, wherein the polyimide-based resin precursor (A) has athermalcuring rate index higher than that of the polyimide-based resinprecursor (B) by not smaller than 10%.
 4. A production method for asubstrate for a flexible print wiring board as set forth in claim 2,wherein the polyimide-based resin precursor (A) has a thermalcuring rateindex higher than that of the polyimide-based resin precursor (B) by notsmaller than 30%.
 5. A production method for a substrate for a flexibleprint wiring board as set forth in claim 1 or 2, wherein thepolyimide-based resin precursor (B) comprises a polyamic acid, or a saltor a mixture of a diamine represented by the following structuralformula (1) and a tetracarboxylic acid derivative represented by thefollowing structural formula (2), wherein the polyimide-based resinprecursor (A) comprises a polyamic acid, or a salt or a mixture of adiamine represented by the following structural formula (1) and atetracarboxylic acid derivative represented by the following structuralformula (2), wherein the content of the tetracarboxylic acid derivativein the polyimide-based resin precursor (A) is greater than the contentof the tetracarboxylic acid derivative in the polyimide-based resinprecursor (B),

wherein R₁ is a tetravalent aromatic residue, R₂ is a divalent aromaticresidue, and n is a real number not smaller than one which represents anaverage number,

wherein R₃ is a tetravalent aromatic residue, and R₄ is a hydrogen atomor an alkyl group.
 6. A production method for a substrate for a flexibleprint wiring board as set forth in claim 1 or 2, wherein thepolyimide-based resin precursor (B) comprises a polyamic acid, atrialkylamine derivative of the polyamic acid, or a mixture of thepolyamic acid and the trialkylamine derivative, wherein thepolyimide-based resin precursor (A) comprises a polyamic acid, atrialkylamine derivative of the polyamic acid, or a mixture of thepolyamic acid and the trialkylamine derivative, and has a greatertrialkylamine content than the polyimide-based resin precursor (B).
 7. Aproduction method for a substrate for a flexible print wiring board, themethod comprising: applying a polyimide-based resin precursor solutiondirectly on an electrically conducting material to form apolyimide-based resin precursor layer; and thermalcuring thepolyimide-based resin precursor layer for production of a flexible printwiring substrate having a polyimide-based resin layer; wherein asolution of a polyimide-based resin precursor (B) comprising a polyamicacid, or a salt or a mixture of a diamine represented by the followingstructural formula (1) and pyromellitic acid orbiphenyl-3,4,3′,4′-tetracarboxylic acid is applied directly on theelectrically conducting material; wherein a solution of apolyimide-based resin precursor (A) comprising a polyamic acid, or asalt or a mixture of a diamine represented by the following structuralformula (1) and pyromellitic acid or biphenyl-3,4,3′,4′-tetracarboxylicacid and containing pyromellitic acid orbiphenyl-3,4,3′,4′-tetracarboxylic acid in a greater amount than thepolyimide-based resin precursor (B) is applied on a layer of thepolyimide-based resin precursor (B).

wherein R₁ is a tetravalent aromatic residue, R₂ is a divalent aromaticresidue, and n is a real number not smaller than one which represents anaverage number.
 8. A production method for a substrate for a flexibleprint wiring board, the method comprising: applying a polyimide-basedresin precursor solution directly on an electrically conducting materialto form a polyimide-based resin precursor layer; and thermalcuring thepolyimide-based resin precursor layer for production of a flexible printwiring substrate having a polyimide-based resin layer; wherein asolution of a polyimide-based resin precursor (B) comprising a polyamicacid, a triethylamine derivative of the polyamic acid or a mixture ofthe polyamic acid and the triethylamine derivative is applied directlyon the electrically conducting material; wherein a solution of apolyimide-based resin precursor (A) comprising a polyamic acid, atriethylamine derivative of the polyamic acid or a mixture of thepolyamic acid and the triethylamine derivative and containingtriethylamine in a greater amount than the polyimide-based resinprecursor (B) is applied on a layer of the polyimide-based resinprecursor (B).
 9. A substrate for a flexible print wiring board producedby a production method as recited in any of claims 1 to
 6. 10. Asubstrate for a flexible print wiring board produced by a productionmethod as recited in claim 7 or 8.